Energy flows in stellar coronae

点击翻译按钮获取中文摘要

The Sun is the only star that we can study in detail, from its interior to the outer atmosphere (corona) and the solar wind, which permeates the entire solar system. The solar corona is also an unique laboratory that we can use to understand fundamental plasma processes that occurr throughout the universe (most of which is composed of magnetised plasma). The solar irradiance and particle flux fundamentally affect space weather, our atmosphere and the global climate of our planet. Yet, we have so far failed to understand many fundamental questions about the Sun (and stars in general). The standard model of the Sun has recently been called into question. We still do not know the prime mechanisms that heat the solar corona to millions of degrees, that cause flares and eruptions, that accelerate particles, that produce the strongest emission in the Extreme Ultra Violet (EUV). We also know very little about the solar irradiance in the EUV or X-rays, and its potential effects on the global climate of our planet. As for other stars, we know that most have strong X-ray emission, however the processes that produce these X-rays are still largely unknown. This is a golden age for the physics of the solar corona, with a wealth of data gathered by recent (SOHO, TRACE, RHESSI) missions, now complemented with Hinode (Solar-B), and soon with STEREO, SDO. The two EUV spectrometers (SOHO/CDS, Hinode/EIS) have a strong UK involvement. I believe that it is timely to invest more resources into the analysis and interpretation of these spectroscopic data, because they hold the key to advance our understanding of the solar corona. The current sensitivity and resolution of EIS will allow novel studies. In particular, I intend to couple observations with theoretical modeling to understand which processes dominate the energy release and flow in the corona, by taking into account non-equilibrium and non-thermal effects in the plasma. In particular, I will study the location and evolution of heating in coronal loops in relation with the changes of the magnetic field, and energy flows during flares, in relation to chromospheric evaporation. Spectral observations and realistic atmospheric models will also shed light on the problem of why the brightest EUV lines are so bright, so in the future we will also be able to predict their emission. I also plan to use the unique 10-year long SOHO observations to characterise for the first time the EUV spectral irradiance and its variations with the solar cycle. We will then be able to study any possible effects on global climate. With the same observations I will also obtain the physical characteristics of the solar corona during the last cycle. With these and some modeling, it will be possible to simulate in detail the X-ray emission from other stars. With these simulations, and Chandra and XMM-Newton observations of stars of different ages, it will be possible to understand in which cases the observed X-ray emission could be due to the presence of a hot corona similar to an active Sun. Provision of accurate atomic data forms an integral part of this proposal, given their importance for plasma diagnostic and modeling. I plan to continue the work on the ions for the EUV and X-rays, and to provide atomic data to allow studies of non-equilibrium and non-thermal effects, which are presumably important in the solar corona.